The present invention is directed to improving diaphragm cells for the electrolysis of aqueous solutions of alkali metal chlorides, for example a sodium chloride brine.
A disadvantage of electrolytic diaphragm cells resides in the difficulty of avoiding the presence of alkali metal chlorate in the alkaline liquor drawn off from the cathode compartments. The presence of alkali metal chlorate in the liquor in fact renders the latter unsuitable for various applications, particularly for the rayon industry, and consequently it necessitates a costly purification of the liquor.
The major part of the alkali metal chlorate present in the alkaline liquor arises from decomposition of hypochlorite ions formed in the anode compartment of the cell, by reaction of the chlorine dissolved in the anolyte with hydroxyl ions coming from the cathode compartment. Besides its unfavourable influence on the presence of chlorate in the alkaline liquor produced in the cell, the formation of hypochlorite ions in the anode compartment gives the disadvantage of also reducing the current efficiency of electrolysis.
In order to reduce the content of chlorate in the alkaline liquor, it has already been proposed, in U.S. Pat. No. 2,823,177 of S. G. Osborne, published Feb. 11, 1958, to incorporate cobalt or nickel in the finely divided state in the diaphragm. In Belgian Pat. No. 773918 of Oct. 14, 1971 in the name of the present applicant it has also been proposed, with the same aim, to incorporate in the diaphragm iron or copper and/or their oxides in the finely divided state.